This book attempts to explain why string theory may provide the comprehensive underlying theory that describes and explains our world. It is an enthusiastic view of how compactified string/M-theories, plus data that may be reachable, seem to have the possibilities of leading to a comprehensive underlying theory of particle physics and cosmology, perhaps soon. We are living in a hugely exciting era for science, one during which it may be possible to achieve a real and true understanding of our physical world. Founded on three decades of research, this second edition covers the Planck scale, Higgs physics, supersymmetry and supersymmetry breaking, the Hierarchy problem, dark matter, inflation, hidden sectors, future colliders, testing theories in physics, possible limits to understanding, and more. Key Features Written by a world-leading authority on compactified string theory Provides simple and informative explanations of many topics Accessible to both physicists and general readers interested in string theory Provides glimpses of future experiments that may provide strong tests of the theory, and experiments that could potentially detect dark matter The second edition includes new material from compactified M-theory identifying the physical inflaton, explaining the matter asymmetry, mass-energy scales of particle physics and cosmology, cosmological histories, dark matter candidates, supersymmetry breaking, future colliders and possible tests.